P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)
The bank 1 catalytic converter no longer stores enough oxygen to clean the exhaust properly.
At a glance
Severity: Low
- Meaning
- Catalyst System Efficiency Below Threshold (Bank 1)
- System
- Emission Controls · Powertrain
- Safe to drive?
- Yes, short term
- Typical repair cost
- $100–$2,800
- DIY difficulty
- Moderate
Your engine computer watches two oxygen sensors: one ahead of the catalytic converter and one behind it. A healthy converter stores and releases oxygen, so the rear sensor gives a fairly steady signal while the front sensor keeps moving as the ECU trims the fuel mixture. When the rear signal starts copying the front one, the ECU concludes the converter's oxygen storage has fallen below its calibrated limit and sets P0420. Bank 1 is the side of the engine with cylinder #1; on a four-cylinder engine it is simply the only bank. The converter is the usual culprit, but a lazy rear sensor, an exhaust leak or an engine problem that ruined the converter must be ruled out before you spend money on a new one.
Can I keep driving? Usually fine to keep driving for now, since the car rarely runs any differently. Stop and get it checked right away if the check engine light starts flashing, the engine loses power or you smell a strong rotten-egg odor: misfires can overheat the converter to the point of melting it.
How to read P0420
- P Powertrain
- 0 Generic (SAE)
- 4 Auxiliary emission controls
- 20 Specific fault
Symptoms
- Check engine light on, usually with no change in how the car drives
- Failed smog or emissions test, or the catalyst monitor stays 'not ready'
- Rotten-egg (sulfur) smell from the tailpipe, especially after hard acceleration
- Rattle from under the car at startup or idle (broken converter substrate)
- Sluggish acceleration or poor top-end power if the converter is plugged
- Slightly worse fuel economy
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Catalytic converter worn out or poisoned by age, oil ash, coolant or silicone
- Rear (downstream) O2 sensor slow, contaminated or reading out of range
- Exhaust leak ahead of the rear O2 sensor (manifold, gasket, flex pipe)
- Engine fault that overheated the converter: misfire, rich mixture, leaking injector
- Engine burning oil or coolant (worn rings, valve seals, head gasket) coating the catalyst
- Low-grade aftermarket converter with too little catalyst for your engine
- Outdated ECU calibration; some manufacturers issued updated catalyst monitor software
Diagnosis step by step
Scan for other codes first
Read all codes and save the freeze frame. Misfire (P0300–P0304), fuel trim (P0171/P0172) or O2 sensor heater codes must be fixed before anything else, because a misfiring engine will destroy a new converter within weeks.
Tools: OBD-II scanner
Graph the front and rear O2 sensors
Warm the engine fully, hold about 2,500 rpm for two minutes, then graph both bank 1 sensors at a steady cruise. A good converter leaves the rear sensor fairly flat around 0.6–0.8 V while the front sensor keeps moving; a rear signal that swings in step with the front one means little oxygen storage is left.
Tools: OBD-II scanner
Check the Mode 6 catalyst result
Most scan tools can display on-board test results (Mode 6). The catalyst monitor value shows how far you are from the limit: a result that only barely fails points you toward a sensor or leak, while one far past the limit usually means a dead converter.
Tools: OBD-II scanner
Hunt for exhaust leaks
With the engine cold, inspect the manifold, gaskets, flex pipe and joints ahead of the rear sensor for black soot trails, and listen for ticking at cold start. A leak pulls in outside air, which the rear sensor reads as extra oxygen and the ECU mistakes for a failed converter.
Tools: Flashlight
Verify the rear sensor responds
Snap the throttle to force a rich condition, then coast in gear with your foot off the pedal to force a lean one. The rear sensor should climb above about 0.8 V and fall below about 0.2 V; if it never gets there or reacts slowly, replace the sensor before condemning the converter.
Tools: OBD-II scanner, Multimeter
Check oil and coolant consumption
Find out how much oil is being added between changes and whether the coolant level drops. Blue smoke at startup or unexplained coolant loss means the new converter will be poisoned too unless that problem is fixed first.
Tools: Flashlight
Test for a restricted converter if power is low
Connect a vacuum gauge to the intake manifold and hold 2,500 rpm. If vacuum slowly sinks instead of holding steady, the exhaust is restricted and the converter is plugged, not just inefficient.
Tools: Vacuum gauge
Where the parts are
- 1Upstream O2 sensor
- 2Catalytic converter
- 3Downstream O2 sensor
- 4Diesel particulate filter
- 5Turbocharger
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace rear (downstream) O2 sensor | Yes | $150–$350 |
| Repair exhaust leak (gasket, flex pipe or manifold) | No | $100–$450 |
| Aftermarket EPA-compliant converter (federal-emissions states) | No | $400–$1,200 |
| OEM or CARB-approved converter (California and CARB-rule states) | No | $1,000–$2,800 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Audi
The second-generation EA888 2.0 TFSI (for example in the B8-generation A4, A5 and Q5) is known for high oil consumption caused by piston rings. Burned oil coats the converter, so confirm oil use before replacing a converter on these cars.
Emissions / smog test impact
Will fail
In states with OBD-based smog or emissions testing, an active P0420 with the check engine light on is an automatic fail. Clearing the code right before the test won't help: the catalyst monitor will show 'not ready', and most states allow only one incomplete monitor on 2001-and-newer vehicles.
Frequently asked questions
Is a P0420 converter covered by warranty?
Often, yes. Federal law treats the catalytic converter as a major emission component covered for 8 years or 80,000 miles, and California-certified vehicles can have longer coverage on some parts. Check your warranty booklet before paying for a converter yourself.
Will a catalytic converter cleaner in the gas tank fix P0420?
Rarely for long. A cleaner can help a converter that is only lightly coated, but it cannot restore catalyst that is melted, cracked or poisoned by oil or coolant. If a bottle makes the light go away, the code usually returns within a few weeks.
Can I use an O2 sensor spacer to get rid of the code?
A spacer moves the rear sensor out of the exhaust stream so it reads a falsely calm signal. It is emissions tampering, stricter states catch it on visual inspection, and the car still pollutes just as much.
Why did P0420 come back after I replaced the converter?
Usually the root cause was never fixed (misfire, oil burning, exhaust leak), or the replacement was a cheap universal unit with too little catalyst for your engine's calibration. Buy a converter listed for your exact vehicle and its emissions certification.
What's the difference between P0420 and P0430?
It's the same test on different sides of the engine. P0420 is bank 1, the side with cylinder #1; P0430 is bank 2 and only exists on engines with two banks, such as V6 and V8 engines.